Editing Ozone layer (section)

==Ultraviolet light==
[[File:Ozone solar UV absorb DNA action.jpg|thumb|300px|UV-B energy levels at several altitudes. Blue line shows DNA sensitivity. Red line shows surface energy level with 10 percent decrease in ozone]]
[[File:Ozone altitude UV graph.svg|thumb|300px|Levels of ozone at various altitudes and blocking of different bands of ultraviolet radiation. Essentially all UV-C (100–280&nbsp;nm) is blocked by dioxygen (from 100–200&nbsp;nm) or else by ozone (200–280&nbsp;nm) in the atmosphere. The shorter portion of the UV-C band and the more energetic UV above this band causes the formation of the ozone layer, when single oxygen atoms produced by UV [[photolysis]] of dioxygen (below 240&nbsp;nm) react with more dioxygen. The ozone layer also blocks most, but not quite all, of the sunburn-producing [[UV-B]] (280–315&nbsp;nm) band, which lies in the wavelengths longer than UV-C. The band of UV closest to visible light, [[UV-A]] (315–400&nbsp;nm), is hardly affected by ozone, and most of it reaches the ground. UV-A does not primarily cause skin reddening, but there is evidence that it causes long-term skin damage.]]

Although the concentration of the ozone in the ozone layer is very small, it is vitally important to life because it absorbs biologically harmful ultraviolet (UV) radiation coming from the Sun. Extremely short or vacuum UV (10–100&nbsp;nm) is screened out by nitrogen. UV radiation capable of penetrating nitrogen is divided into three categories, based on its wavelength; these are referred to as UV-A (400–315&nbsp;nm), [[UV-B]] (315–280&nbsp;nm), and [[UV-C]] (280–100&nbsp;nm).

UV-C, which is very harmful to all living things, is entirely screened out by a combination of dioxygen (< 200&nbsp;nm) and ozone (> about 200&nbsp;nm) by around {{convert|35|km|ft}} altitude. UV-B radiation can be harmful to the skin and is the main cause of [[sunburn]]; excessive exposure can also cause cataracts, immune system suppression, and genetic damage, resulting in problems such as [[skin cancer]]. The ozone layer (which absorbs from about 200&nbsp;nm to 310&nbsp;nm with a maximal absorption at about 250&nbsp;nm)<ref>{{cite journal |title=Photolysis of Atmospheric Ozone in the Ultraviolet Region |author1=Matsumi, Y. |author2=Kawasaki, M. |journal=Chem. Rev. |date=2003 |volume=103 |issue=12 |pages=4767–4781 |pmid=14664632 |doi=10.1021/cr0205255 |url=http://yly-mac.gps.caltech.edu/N2O/Prasad/Matsumi_O3_cr0205255%20copy.pdf |access-date=March 14, 2015 |url-status=dead |archive-url=https://web.archive.org/web/20120617123007/http://yly-mac.gps.caltech.edu/N2O/Prasad/Matsumi_O3_cr0205255%20copy.pdf |archive-date=June 17, 2012}}</ref> is very effective at screening out UV-B; for radiation with a wavelength of 290&nbsp;nm, the intensity at the top of the atmosphere is 350 million times stronger than at the Earth's surface. Nevertheless, some UV-B, particularly at its longest wavelengths, reaches the surface, and is important for the skin's production of [[vitamin D]] in [[mammals]].

Ozone is transparent to most UV-A, so most of this longer-wavelength UV radiation reaches the surface, and it constitutes most of the UV reaching the Earth. This type of UV radiation is significantly less harmful to [[DNA]], although it may still potentially cause physical damage, premature aging of the skin, indirect genetic damage, and skin cancer.<ref>{{cite journal |title=Review: Ultraviolet radiation and skin cancer |author1=Narayanan, D.L. |author2=Saladi, R.N. |author3=Fox, J.L. |journal=International Journal of Dermatology |volume=49 |issue=9 |pages=978–986 |date=2010 |pmid=20883261 |doi=10.1111/j.1365-4632.2010.04474.x |doi-access=free |s2cid=22224492}}</ref>